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CLIC damping ring vacuum requirements and coating

CLIC damping ring vacuum requirements and coating

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CLIC damping ring vacuum requirements and coating

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  1. CLIC damping ring vacuum requirements and coating G.Vandoni TE/VSC G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  2. CLIC DR wiggler vacuum chamber for ANKA-IMAGE G.Vandoni TE/VSC G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  3. Outline • DR vacuum and surface issues • Vacuum/surface requirements for DR wiggler • Synchrotron radiation and photodesorption yield • Cold vacuum issues • Conductance and pumping • SR and vacuum in ANKA and CLIC DR • Other requirements: mechanical tolerances • Technological challenges • Possible experimental program and set-up in ANKA • Planning/Budget • Conclusions G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  4. Electron cloud issues • e-cloud is not a problem in the electron ring: • not for heat load • not for multipacting • e-cloud is a critical problem in the positron ring: • beam stability • multipacting • heat load • Secondary Electron Yield critical threshold: 1.3 • Primary electrons are essentially photoelectrons SR radiation G.Rumolo , private communication G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  5. Vacuum issues – electron ring Fast Ion Instability: Fast ion instability, results from positive ions (critical mass ~13) getting trapped in a bunch train Ions like CO+, N2+, H2O+ are concerned. It results in a ion cloud at the end of the train. Ions come from the residual gas. Calculation in the case of the CLIC DR assuming vacuum of 10-9 mbar has shown a very fast rise time for the ion instability: 1.1ms. Hence a vacuum better than10-10mbar is requested. Collective effects in the CLIC Damping Rings, G.Rumolo et al. EPAC08 Genoa Residual gas: Thermal outgassing, photon, electron and ion induced desorption. SR radiation G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  6. Vacuum/surface DR wiggler requirements ELECTRON RING UHVacuum 10-11mbar bakeout in-situ NEG stripes or NEG coating, activation in situ POSITRON RING Mitigation of e-cloud: SR absorption and reduction of SEY amorphous carbon coating NEG coating, activation in-situ controlled roughness COMMON REQUIREMENTS Impedance reduction Mechanical stability and tolerance Power deposition on cold vacuum conductive chamber or coating extremity smooth transitions, with RF fingers impedance reduction for beam stability and limit power deposition G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  7. Synchrotron radiation- directional and power distribution – in CLIC DR SR distribution starting from the 6th wiggler, alternating even and odd Synchrotron radiation is directed zenithally: the azimuthal components being screened by the successive absorbers. 1W/m 20W/m Critical energy: 13.5keV H2photodesorption originates in the last 50cm, at the roof and floor of the vacuum chamber. courtesy D.Schoerling, CDR draft G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  8. SR induced desorption from NEG film Photodesorption yield from non activated NEG is low (e.g. 2.10-3 at 1021 photons m-1) with respect to stainless steel. It decreases by 2 orders of magnitude for activated NEG Critical energy: 20.5keV Photon flux: 2.9.1015s-1mA-1 e- beam current: 185mA P.Chiggiato et al. Vacuum, 2000 G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  9. SR induced desorption from amorphous carbon films Photodesorption yield is lower (between 1 and 5.10-4 at 1021 photons m-1) from non baked a-C film than for baked stainless steel. Critical energy: 20.5keV Photon flux: 2.9.1015s-1mA-1 e- beam current: 185mA P.Chiggiato, private comm. G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  10. Cold vacuum issues • In the NbTi alternative, the magnet may be cooled at 4.2K by a LHe heat exchanger. • The vacuum chamber, inserted in the insulation vacuum vessel, would be cooled independently, via cooling channels or distributed heat exchanger with one of the thermal screens (20K or 80K). • Advantages: • full use of magnetic gap, downgrade of mechanical resistance requirements • Vacuum chamber of variable temperature… • Some points for discussion • Cryopumping? Vapour pressure curves (next slide) • You said Beam Screen? conduction-cooled (dry) magnet available vertical distance for vacuum chamber (thickness, coatings, roughness, and thermal insulation): 2.5mm courtesy D.Schoerling DRAFT G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  11. Effect of vapor pressure on residual gas limit pressure →For H2, cold vacuum is not useful above 2K CO, CO2 killed at 20K, but not at 80K courtesy V.Baglin G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  12. Digression: what is a beam screen (BS) cold bore, 1.9K Cryogenics Dynamic heat load is intercepted with better performance (i.e., extraction of heat at cold expending work) at intermediate temperature. slotted beam screen, 5K-20K saw-tooth to limit SR reflection W/Qc=energy spent to extract heat at cold, Watts/Watt, doubles between 4.2Kand1.8K heat evacuation by separate gas flow or heat intercepting • Vacuum • A 1.9K cold bore is a higly efficient cryopump with infinite capacity. • Having a BS for cryogenics, vacuum uses it for: • screening the cryopumping cold bore against primary desorption from the beam (SR) • recycling and evacuating gas cryosorbed at the BS and desorbed by SR: more effective than at 2K, because requiring less energy • reducing photon reflectivity by a sawtooth structure imprinted on the copper inner coating in the azimuthal plane G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  13. Do we need/ can we afford a beam screen? We have seen that a beam-screen screens a 100% effective (cryo)pump against primary desorption phenomena. And being the surface seen by the beam, it may also be used as a support for coatings and structurization, e.g. for photon reflection mitigation. Without a cryopump, no need for a beam-screen for vacuum. With a 4.2K magnet, cryopumping is not efficient for H2. Even more so if the vacuum chamber is ~20K (N2) DO WE NEED IT? Vacuum doesn’t need a beam screen A beam screen would further reduce the aperture. It would increase impedance. It would need a conductive coating to reduce wall resistivity. Difficult trade-off between aperture and mechanical resistance, conductive coating, roughness for photon absorption. CAN WE AFFORD IT? Does cryogenics need a beam screen? Not with a 20K vacuum chamber BASELINE NOW: no beam screen G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  14. Vacuum conductance and pressure profile Longitudinal conductance Gas load With an approx. photodesorption yield=10-3 We can estimate the H2 flux with 20W/m SR power, hence the gas load to evacuate. PRESSURE PROFILE C=1.9 l/s Conductance of 2m chamber at 20K Q: desorbing flux [mbar.l.s-1] S: pumping speed [l/s] C: tube conductance [l/s] Option 1: Localized pumping at the extremities a b A very large localized pumping speed is required to limit P1<10-10mbar Option 2: Distributed pumping G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  15. Distributed pumping NEG stripes ZrVFe: Activation at 300oC for 24h OPEN ISSUE LEP vacuum chamber NEG film coating TiZrV : Activation at 180oC for 24h Activated NEG film provides also SEY reduction and it reduces photodesorption By a judicious choice of the NEGs, we can switch ON/OFF each The form should result from a trade-off between impedance, cryostat dimensions, volume of NEG, etc. LHC LSS vacuum chamber G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  16. Synchrotron radiation and vacuum: ANKA/CLIC DR main differences ANKA CLIC DR Azimuthal Distributed? Zenithal Localized Angular distribution Longitudinal distribution SR in ANKA originates from the preceding Bending Magnet. A scraper may absorb a part of it or be opened to increase power on the vacuum chamber. An antichamber to absorb SR may be interesting for ANKA, not for CLIC DR 90o Vacuum problems (desorption region) and mitigation methods (SR absorption and SEY reduction) are turned by 90o Vacuum would require an antichamber or a large chamber, but not for SR absorption G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  17. Dimensions and tolerances –tentative- Maximal thickness depending on cryostat design and cooling scheme, as well as thermal insulation with respect to magnet and shimming or supporting towards magnet, To be confirmed TOLERANCES Vacuum Chamber: 100 mm Magnet: 50 mm G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  18. Technological challenges for coatings Graphite rod • Coating by magnetron sputtering • (Fig.: amorphous carbon) • Long thin rod or • Displacement of electrode in chamber • Sufficient distance with to-be-coated support noble gas ion Electron C atom + + + + + + NEG Courtesy M.Taborelli + + a-C + Variable parameters -gas pressure -power • Difficult with NEG (8mm minimal chamber diameter, coated at ESRF), trial & error , 3 thin cathodes 2mm diameter one besides the other to ensure uniform coating. • Requires R&D for a-carbon within 13mm: thick slab of graphite • Difficult to move the electrode longitudinally with perfect stability for a uniform coating -700V B-field Feasability tests at CERN G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  19. Technological challenges for activation Activation and bake-out Bakeout to decrease outgassing rate and/or activation of a NEG coating requires heating at 180oC, under vacuum. Is this compatible with a surrounding wiggler? Heater: layered assembly of electrical insulator (Kapton), resistive heater and reflective foil, wrapped around the vacuum chamber in a very small thickness (0.6mm). In addition, thermocouple and electrical connections. Wrapping of heater around LHC LSS vacuum chambers Can the cryostat + wiggler withstand heating of the vacuum chamber up to 180oC? Thermal insulation between wiggler and vacuum chamber against cold+ warm temperature. Feasability tests at CERN G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  20. What could we explore in ANKA 1 NEG stripes and NEG coating for electron-ring: VACUUM: Reduction of vacuum to the required value 10-11mbar, pressure profile NEG film pumping under SR with variable temperature Activation of NEG film in-situ, with closed cryostat Activation of NEG stripes Reduction of photoelectrons 2 1 vacuum chamber, with NEG stripes and NEG film coating Amorphous carbon coating for positron ring: VACUUM: Vacuum behavior of amorphous carbon in SR: photodesorption, outgassing, DIFFICULTY with the effect of surrounding chambers Reduction of photoelectrons 1 vacuum chamber, with NEG stripes and amorphous carbon film coating VACUUM: requires instrumentation as well as particular vacuum conditions Electrons: what type of instrument? OPEN ISSUE for discussion G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  21. Vacuum instrumentation and conditions • Vacuum Instrumentation • Residual Gas Analyzer RGA X-rays screened with Pb shield • Total pressure calibrated measurement • Boundary conditions • Adjacent warm vacuum should not modify the measurement !! • To isolate the wiggler’s vacuum effect from adjacent surfaces, the rest of the straight section should be warm and entirely NEG-coated, on both sides (±2m). Cold-warm transitions Vacuum instrumentation should be as close as possible to the wiggler Scraper, interfaces wiggler 0.5m 0.5m 2m 0.5m 0.5m G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  22. (Tentative) experimental programme Start with vacuum chamber with oxidized NEGs (non activated) Activate the NEG film at 180oC Activate the NEG stripes at 300oC VACUUM with NEG Comparative Vacuum Measurements VACUUM with a-C Start with vacuum chamber with oxidized NEGs stripes (non activated) Activate the NEG stripes at 300oC And parasitic electrons? G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  23. Planning G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  24. Interfaces of vacuum chamber - to be defined Smooth transition to ANKA beam pipe to reduce impedance Type of flange and RF connection, control of gaps Extremity Cryostat Thermal insulation towards magnet: reflective foil or Multi-layer insulation Mechanical supports or shims Thermal sink to thermal screen or Cooling channels Electrical feedthroughs for heaters, heating current to NEG stripes, thermocouple G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  25. Conclusions • A vacuum chamber in stainless steel with a large transverse dimension or antichamber • NEG stripes ZrVFe located oppositely to SR • NEG coating • wrapping for heater • Preference goes to a dry magnet, to get thickness for heater, coatings, heat transmission and thermal insulation • Feasibility studies include • Thin film magnetron sputtering production in small vertical aperture • Thermal insulation of vacuum chamber against wiggler for activation of NEG • An experimental program with limited number of vacuum-chamber changes • NEG-coated adjacent chambers (cold-warm transition, warm interfaces) to limit influence of warm vacuum on the measurement • Residual Gas Analyzer, Pb-shielded against X-rays G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany

  26. Open issues Vacuum chamber transverse dimension/ form/ material Parasitic electron detection Compatibility of in-situ activation with cryostat and wiggler Feasability of coating in limited vertical aperture of the vacuum chamber Impedance considerations …. G.Vandoni @ Experimental Program for CLIC Damping Wiggler Prototype Test at ANKA, Germany